Evidence against a role of general protein kinase C downregulation in skin tumor promotion.

Using isoenzyme-specific antibodies, we have performed an immunoblot analysis of the PKC isoenzyme pattern during the course of TPA-induced tumor promotion in the epidermis of NMRI mice. The TPA-sensitive PKC isoforms alpha, delta, straightepsilon, eta, nu (and TPA-insensitive PKCzeta), but not PKCbeta and gamma, were found to be expressed in both normal and neoplastic epidermis. The immune signals of all TPA-sensitive PKC isoforms were moderately and reversibly attenuated upon a single TPA treatment. Using different antibodies against PKCeta and PKCmu, this apparent downregulation could mainly be attributed to epitope changes of these enzymes, whereas for the other PKC species no such conclusion could be drawn. Except for PKCstraightepsilon, no substantial long-term attenuation of the immune signals of the other PKC isoforms occurred upon chronic phorbol ester treatment (i.e., 14 applications of 5 nmol TPA each over 7 weeks), which led to tumor development in initiated mouse skin. Specific PKC activity (related to tissue weight) was 40-50% lower in TPA-treated as compared with control epidermis whereby no clearcut difference was found between single and chronic TPA treatment. Benign and malignant skin tumors generated according to the initiation-promotion protocol did not exhibit consistent alterations in the immune pattern of the PKC isoenzymes with the exception of a decrease of PKCstraightepsilon and an increase of PKCmicro signal in carcinomas. Our data indicate that, in contrast with earlier assumptions, no general long-lasting PKC downregulation plays a critical role in skin tumor promotion. Copyright 2000 Wiley-Liss, Inc.